1. Field of the Invention
The invention concerns a method for producing a three-dimensional object by means of a stereolithographic process, including the computer graphic design of the object and in particular of the joining elements of the supports that connect the different parts of the object with each other.
The invention concerns also a method for the computer graphic design of the object and in particular of said joining elements.
It is known that stereolithography is a technique that allows three-dimensional objects to be made starting directly from digital data processed by a CAD/CAM software.
2. Present State of the Art
Stereolithography is used mainly to make prototypes, as it makes it possible to obtain, in a short time, three-dimensional objects, even with a complex structure, which can be analysed and tested before proceeding to the industrial production stage.
For this purpose, the stereolithography process is widely used to make the models of objects that will be successively produced through investment casting.
The stereolithography process substantially produces three-dimensional objects through the superimposition of a plurality of layers with thickness in the order of tenths of micrometers, which are obtained by exposing a liquid resin that polymerises in the presence of light to selective light stimulation in the areas corresponding to the volume of the object to be produced.
The market offers a plurality of types of stereolithography machines, among which, for example, there are stereolithography machines substantially comprising a tank suited to contain the liquid resin and delimited by a transparent bottom and by a modelling head.
The modelling head is positioned on top of the tank and is provided with a plane surface suited to support the three-dimensional object being formed and power means that move the plane surface with respect to the bottom of the tank.
At the beginning of the object formation process the plane surface is immersed in the liquid resin until it comes to be positioned at a distance from the bottom that is equal to the thickness of the first layer to be obtained.
The resin layer included between the bottom of the tank and the plane surface of the modelling head is selectively exposed to a laser beam coming from a laser light emitter arranged under the bottom of the tank and associated with a Cartesian coordinate robot.
The resin is thus exposed to electromagnetic radiation only in the areas corresponding to the volume of the object to be made and during solidification it adheres to the plane surface of the modelling head.
Once the solidification process has been completed, the head is lifted together with the first layer of the object that adheres to it, in order to allow the level of the resin to be restored inside the tank.
Once the resin level has been restored, the head is immersed again in the resin and the operations described above are repeated in order to deposit a second layer that will adhere to the first one and so on, until the object to be made has been completed.
If the object includes portions that project from or are undercut in the base and that may yield or bend, during the object formation process special columns, commonly called supports, are created to support said projecting or undercut portions, wherein said supports constitute an integral part of the object and are removed once the latter has been completed.
According to the known technique, the removal of the supports, a large number of which may be present in particularly complex objects, is performed manually by means of a cutter and poses several acknowledged drawbacks.
If the object is used as a model to obtain parts that are cast by means of the investment casting technique, the manual removal of the supports from the model cannot guarantee a constant high quality of the finished piece.
In fact, if the removal of the supports is performed by cutting them in a position that is excessively spaced from the surface of the model, once the metal has solidified, corresponding projecting stumps that must be removed through mechanical processing will be present on the part that is successively produced through investment casting.
If, on the other hand, the supports are removed by cutting them at the level of the surface of the model, the cutter blades, as is known, in any case create a recess in the body of the model and in each cutting point, so that a void area that must be properly filled will correspond to said recess in the solidified metal part.
It can thus be understood that in practice the impossibility for the operator of cutting manually, in the same way, all the supports present in the model means that projections and hollows distributed at random will be present on the finished piece.
Therefore it can be noticed that it is practically impossible to make cast pieces having all the same quality.
Another drawback lies in that since the cutting of the supports depends only on the ability of the operator in maneuvering the cutter, execution times and costs are rather high.
A further drawback is represented by the fact that with the same resistant section the cutting stress that is necessary to cut each support varies as the type of material with which the support is made varies.
Thus, in order to make supports that can be cut with efforts having a constant value or a value that can be determined by the operator, it would be necessary to design and make them with different cross sections depending on the type of material used.
According to the state of the art, a method is known that is used to make supports for three-dimensional objects by means of a stereolithography process and that is described in the patent document US 2009/0072447 A1.
The indications given in said patent document are intended for the design of the sections of the supports that bear the three-dimensional object during its formation process.
More particularly, the indications contained in the patent document are limited to the definition of a method suited to determine the optimal geometry of the supports so that they are sufficiently resistant to support the three-dimensional object being formed.
The above mentioned patent document, therefore, does not contain indications useful to design and define in an analytical manner the geometry of the joining elements that connect the supports to the three-dimensional body, and thus the indications contained in the above mentioned patent document do not make it possible to overcome the drawbacks of the known art that have been described above.